What I'm I doing wrong? Can't you have two 'o=' attributes in a DN? And
why does it complain about a missing attribute 'o', when in fact is looks
to be there?

To have multi-valued attributes, you list them twice:

o: client
o: companyName

I think you don't understand the o/ou hierarchy though, since I can't see
how an "o" would appear below an "ou"...

Also, the correct list for general LDAP questions is "ldap@umich.edu".
This list is for questions specific to the OpenLDAP software, so if you
have further generic LDAP issues, I advise posting to the ldap@umich list.